The thermal dehydration of MgSeO4 . 6H(2)O, MgSeO4 . 4H(2)O and MgSeO4 . 2H(2)O has been studied by TG, DTA and DSC. The dehydrations occur in steps and the intermediate hydrates MgSeO4 . 5H(2)O, MgSeO4 . 4H(2)O, MgSeO4 . 2H(2)O and MgSeO4 . H2O are produced. The enthalpies of dehydration of the observed stages have been determined. The enthalpies of formation of MgSeO4 . 6H(2)O, MgSeO4 . 4H(2)O, MgSeO4 . 2H(2)O and MgSeO4 . H2O have been calculated from the DSC data.